There have been widely used paper food containers and plastic food containers for weight saving, as well as reduction of production and distribution costs of containers. Traditionally, polyethylenes have been used for imparting heat-sealing properties to the innermost layer of paper containers and plastic containers. Polyethylenes have excellent heat-sealing properties, and exhibit superior film moldability upon laminating with paper or plastic, which contributes to easy processing and good productivity. For this reason, polyethylenes have been used for a variety of purposes in this field.
In the meantime, due to the widened taste of consumers toward natural fruit juices and alcoholic beverages to be contained in paper and plastic containers, a sealant superior to polyethylenes has been demanded. The reason for such demand is that while polyethylenes have the above-mentioned superior properties, they have defects in that they adsorb flavor of beverages, cause change of the taste and/or flavor of the beverages, and lose original flavor of the beverages through migration of odor peculiar to the polyethylenes into the beverages. When compared with other containers such as glass bottles and PET bottles, polyethylenes are poor in flavor retention, and an improvement in this aspect has been strongly demanded.
On the other hand, polyester resins show good flavor retention as compared with polyethylenes, and their use as heat-sealants for food containers has already been proposed in, for example, Japanese Unexamined Patent Publication Nos. 206859/1985 and 81042/1988. While polyester resins are superior in heat-sealing properties and flavor retention, when melt-extruded on paper or a plastic film, they accompany great neck-in at the bottom of T-die. Therefore, a high-speed molding into a film which is available with polyethylenes is extremely difficult with polyesters, which can result in poor productivity.